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This paper presents conceptual models of how density dependence and regulation operates in populations. Limitation is a process which sets the potential equilibrium level, and is caused by all forms of mortality or reproductive loss. Random variations in these mortalities will cause the population to fluctuate about the equilibrium. Regulation is the tendency of the population to return, through density dependent factors, to the equilibrium level when disturbed from it. The strength of density dependence determines whether a stable equilibrium, limit cycle or chaotic behaviour occurs. A population rarely, if ever, remains at its equilibrium because of disturbances. Because the amplitude of population fluctuations is determined by both the strength of density dependence and the size of the density independent mortalities, knowledge of both are required to understand population dynamics. Compensation is the ability of a mortality factor to counteract the effects of another one and requires a change in strength of the density dependence. Exact compensation rarely, if ever, occurs. Predators can act in a density dependent way to hold prey numbers at low levels. They can also act in an inverse density dependent (depensatory) way, which at high prey densities limits but not regulates the prey, but at low densities may cause the extinction of prey. Predation can produce special cases of multiple stable states and stable limit cycles. We discuss some of the criticisms of density dependence.
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